This invention relates to molten media and in particular to a method and apparatus for quiescently moving or urging a molten media such as a molten metal or a molten salt through a system such as a scrap reclamation system wherein the molten media may be used to dissolve or melt metal scrap.
It is generally recognized that pumping or moving a high temperature molten media such as molten aluminum or a molten salt through any system or process normally requires expensive pump equipment because of the temperatures involved. Often the high expense associated with such pump equipment occurs because of the precision and close tolerances employed to prevent leaking of the molten media at the pump drive shaft and consequent binding upon solidification or freezing of the escaping molten media. A further reason for high cost pump equipment when molten aluminum is involved results from the selective material from which the pump must be fabricated to avoid the corrosion or erosion effects that molten aluminum has on various metals thus necessitating the use of refractory materials which are not only expensive but are less durable than metal.
Because of the close tolerances and refractory materials employed in many molten metal pumps, pump breakdowns occur with alarming regularity. Not only is the pump itself expensive to repair, but repairs require substantial time periods during which the system dependent on the pump must also be interrupted. This problem is particularly acute in recirculating systems for melting and reclamation of metal scrap. The recirculating system or process referred to is one which utilizes a hot molten melting media to melt the metal scrap. It includes a zone where heat is applied to a melting media and a melting bay to which the heated melting media is circulated to melt metal scrap, the melted scrap and melting media being then returned to the heating zone and molten metal removed at a rate commensurate with the charging rate. A pumping or circulating means is an essential component of any such system. If in such a system the pump jams, wedges or otherwise stops, the incoming cold metal scrap will quickly cause the melting media to freeze or solidify and consequently require a great deal of expense and labor to restart the process. Subsequent stations in the molten metal system such as metal degassing, purification, and ingot casting or other treatments are also interrupted such that the disruptive effect of a pump breakdown is obviously a serious problem in a molten metal system.
A further problem encountered in a recirculating molten aluminum process used to reclaim aluminum containers is the formation of skim. One form of skim referred to in recirculating molten aluminum processes is the oxide which aluminum in its molten state forms when it is exposed to air. Obviously, therefore, in this type of process no more surface area of molten aluminum should be exposed to the atmosphere than is necessary. For example, surface turbulence, cascading or vortexing of molten aluminum can themselves constitute a source of skim generation and often have to be kept to the lowest possible level. The oxide skim entrains unoxidized metal in substantial amounts as it rises from the metal melt to form a floating skim layer and this entrained metal further contributes to the melt loss associated with skim. The circulatory melting systems referred to above are especially sensitive to skim formation since floating skim acts as an insulator and substantially reduces the ability to heat the molten metal through its surface. The reduced heat input to the molten metal obviously reduces the melting capacity of the circulatory system, and in addition, equipment life tends to be shortened by absorbing heat rejected by the insulative oxide skim. Hence, an additional burden placed on any pumping means employed in such a system is that it should not aggravate the skim problem.
A system for moving molten media through an industrial process, i.e. a recirculating metal scrap reclamation process, has been discovered which solves all of these problems by being relatively inexpensive, easily and quickly repaired without requiring shutdown of the process, and relatively skim-free when used to recirculate molten aluminum in a process for reclaiming used aluminum containers such as food and beverage containers. Furthermore, this system is especially suited to existing level pour molten metal transfer systems which are widely used throughout the light metals industry, particularly the aluminum industry since aluminum in its molten state, as was mentioned earlier, is very reactive with oxygen.